Computer-aided osteoplasty surgery system

ABSTRACT

A method for performing computer-assisted orthopaedic surgery includes the steps of: ( 1 ) producing and displaying three-dimensional geometrical models of first and second bones, the first and second bones forming a joint; ( 2 ) identifying a zone of impingement between the first bone and the second bone on at least one of the bones; and ( 3 ) generating and displaying a color map of at least one surface of at least one bone, the at least one surface being within the zone of impingement, the color map including different colors representing different depths of bone to be removed in order to achieve an increased range of motion between the first and second bones.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.11/688,628, filed Mar. 20, 2007, which claims the benefit of U.S. patentapplication Ser. No. 60/784,639, filed Mar. 21, 2006, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

This invention relates to the field of computer assisted orthopaedicsurgery, and more particularly, to osteoplasty surgeries, such asFemoral Acetabular Impingement surgery.

BACKGROUND

Osteoarthritis (OA) is thought to be caused by a combination ofintrinsic vulnerabilities of the joint, such as anatomic shape andalignment, and environmental factors, such as body weight, injury, andoveruse. In the hip, for example, it has been postulated that much ofosteoarthritis is due to developmental anatomic deformities. Recentstudies have demonstrated that more subtle anatomic abnormalities, suchas acetabular retroversion, acetabular over-coverage, and decreasedhead-neck offset of the femoral head-neck junction are also importantanatomic variants of the hip joint that may lead to pain and OA.

Femoral Acetabular Impingement (FAI) is often classified into twodistinct entities, namely, cam impingement and pincer impingement. Camimpingement results from pathologic contact between an abnormally shapedfemoral head and neck with a morphologically normal acetabulum. Thispattern of impingement is characterized by a femoral head-neck junctionwhich is not spherical anteriorly and has increased radius of curvature.As the hip flexes, this abnormal region engages the anterior acetabulum.The resultant shear forces which result from this contact produces thecharacteristic anterosuperior chondral injury and associated labraltear. The second type of FAI, pincer impingement, is the result ofcontact between an abnormal acetabular rim and a typically normalfemoral head-neck junction. This pathologic contact is the result ofabnormal anterior acetabular “over coverage.” This results in decreasedjoint clearance and repetitive contact between the femoral neck andacetabulum. Ultimately, this repetitive contact causes degeneration ofthe anterosuperior labrum much like in cam impingement. The injuredlabrum subsequently may become calcified, further worsening the anterior“over coverage.” Additionally, because the anatomic constraint in thenative hip is so great, the contact can cause leverage of the head outof the acetabulum posteriorly contributing to a “contre-coup” injury tothe posteroinferior acetabulum.

Cam and pincer impingement differ in mechanism, epidemiology,pathoanatomy and surgical management. However, it is not uncommon to seeboth of these lesions coexisting in a patient with FAI.

The goal of surgical intervention is to relieve the impingement byincreasing hip clearance in flexion or some other motions as well asaddressing the associated labral and chondral pathology. Surgery istailored to the underlying anatomic abnormality. Cam type impingement,with prominence of the femoral head-neck region, is addressed on thefemoral side with femoral neck osteoplasty or osteochondroplasty. Thegoal of femoral neck osteoplasty is to recreate the anatomic sphericityof the femoral head and to reduce the prominence of the femoral neckwhich abuts the anterior labrum and acetabulum. Conversely, pincerimpingement lesions often require resection osteoplasty of theacetabular rim with repair of the labrum to its proper anatomicposition. When these lesions coexist, osteoplasty of both the femoralhead-neck junction and the acetabular rim is required.

Classically, the surgical approach to these lesions has been a formalopen surgical dislocation including trochanteric osteotomy. Thisapproach has been espoused for its ability to give an unobstructed360.degree. view of the femoral head and acetabulum.

Minimally invasive surgery (MIS) and arthroscopic techniques forosteoplasty cause much less morbidity and pain for the patient, and helpto promote a much quicker recovery. However, they are significantly moredifficult and even impossible for some surgeons to perform, mainly dueto the reduced visualization and access in comparison to opentechniques. Identifying the impingement zones is problematic due to, forexample, the flexion of the hip joint and the interference ofsurrounding soft tissues. Moreover, it is extremely difficult tovisualize the correct amount of bone that should be removed, and toeasily verify this without removing too much bone.

In MIS, fluoroscopy is often needed and used to enhance visualization.However, introducing a fluoroscopic arm into the operating room places agreat burden on the surgeon, operating room staff, and patient due tothe logistic, ergonomic, and radiation issues.

Some have advocated simulating the patients' range of motion onpre-operative images such as computed-tomography (CT) scans or MRI, inorder to the physician in planning surgery. However, this approach hasseveral disadvantages. Pre-operative scans such as CT scans requirecostly imaging equipment and technicians, and they are time consumingfor the surgeon. These simulation tools require the transfer andprocessing of images, using segmentations algorithms which are often notrobust and difficult to use. In addition, the simulations cannot takeinto account the actual kinematics of the patient, and the effects ofsoft tissues on the patient's real range of motion. The surgeon is alsoburdened with having to register this preoperative plan to the patientin surgery.

Therefore, an intraoperative tool to help surgeons plan and performoseteoplasty surgeries, such as, FAI in a more precise and less invasivemanner would be an invaluable tool for surgeons and for patients.

SUMMARY

According to one embodiment, a method for performing computer-assistedorthopaedic surgery includes the steps of: (1) using a device to acquirefirst coordinate points on a surface of a first bone; (2) using thedevice to acquire second coordinate points on a surface of a second bonethat forms a joint with the first bone; (3) producing and displaying athree-dimensional geometrical surface model of the first bone based atleast initially on the acquired first coordinate points; (4) producingand displaying a three-dimensional geometrical surface model of thesecond bone based at least initially on the acquired second coordinatepoints; (5) moving the first and second bones relative to one anotherand detecting maximum amplitudes of rotation between the first andsecond bones; (6) identifying a zone of impingement between the firstand second bone on at least one of the bones; (7) displaying as a colormap at least one surface of at least one bone model, the at least onesurface being within the zone of impingement, the color map includingdifferent colors representing different depths of bone to be removed inorder to achieve an increased range of motion between the two bones; (8)using a tracked tool to remove bone in the zone of impingement based onreal time information provided on the color map; and (9) moving againthe first and second bones relative to one another and detecting anincreased amount of rotation between the first and second bones.

In another aspect, a method for performing computer-assisted orthopaedicsurgery includes the steps of: (1) producing and displayingthree-dimensional geometrical models of first and second bones, thefirst and second bones forming a joint; (2) identifying a zone ofimpingement between the first bone and the second bone on at least oneof the bones; and (3) generating and displaying a color map of at leastone surface of at least one bone, the at least one surface being withinthe zone of impingement, the color map including different colorsrepresenting different depths of bone to be removed in order to achievean increased range of motion between the first and second bones.

These and other aspects, features and advantages shall be apparent fromthe accompanying Drawings and description of certain embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a perspective view of a computer-assisted orthopaedic surgery(CAOS) system according to one embodiment of the present invention;

FIG. 2 is a perspective view of an image of a femur bone showing theacquisition of data points along the femur using a pointer or the likeof the CAOS system to provide a model of the femur using bone morphingtechniques;

FIG. 3A is a cross-sectional view of an image of a femur bone in a firstposition relative to the acetabulum surface of the pelvis;

FIG. 3B is a cross-sectional view of the femur bone moved to a secondposition relative to the acetabulum;

FIG. 3C is a cross-sectional view showing the femur bone is a series ofdifferent positions relative to the acetabulum;

FIG. 3D is a cross-sectional view showing the relationship between anybone to point to the hip center;

FIG. 4 is a perspective view of an image of a femur with a probe beingused to define a potential impingement zone;

FIG. 5 is a perspective view of an image of a femoral bone surface, inreal time, as it is being milled, with color map information displayedon the newly burred surface;

FIG. 6 is a perspective view of the acetabulum surface showing theacquisition of a number of data points thereon using a pointer or thelike of the CAOS system to provide a model of the acetabulum using bonemorphing techniques;

FIG. 7 is a perspective view of an image of the acetabulum surface witha shaving tool being navigated therealong for shaving a select, targetportion of the acetabulum surface;

FIG. 8 is an image of the leg being placed in a neutral position forsubsequent movement of the hip joint to check the amplitude; and

FIG. 9 is a perspective view illustrating real-time milling of the femurwith clock reference.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION

The system according to one exemplary embodiment of the presentinvention is made up of a number of working components that interactwith one another to perform a number of different operations. Theheadings below are merely for highlighting the various components of thepresent system, as well as the various operations or tasks performed bythe system. As such, the headings are not limiting of the scope of thepresent invention.

Camera and Tracking

Referring now to FIG. 1, a computer-assisted orthopaedic surgery (CAOS)system 10 is schematically shown. The CAOS system 10 is configured forperforming joint preserving surgeries, such as osteoplasties and femoralacetabular impingement surgery. The system includes a suitable positionmeasuring device 20 that can accurately measure the position of markingelements in three dimensional space. The position measuring device 20can employ any type of position measuring method as may be known in theart, for example, emitter/detector or reflector systems including optic,acoustic or other wave forms, shape-based recognition trackingalgorithms, or video-based, mechanical, electromagnetic and radiofrequency systems. In an embodiment, schematically shown in FIG. 1, theposition measuring system 20 is an optical tracking system that includesat least one camera 49 that is in communication with a computer system30 and positioned to detect light reflected from a number of speciallight reflecting markers or spheres shown at 50.

Detecting and determining the position and orientation of an object isreferred to herein as “tracking” the object. To provide precisiontracking of objects, markers can be rigidly connected together to formreference bodies, (e.g., 100, 110), and these reference bodies can beattached to bones, tools and other objects to be tracked. One suchdevice that has been found to be suitable for performing the trackingfunction is the Polaris™, system from Northern Digital Inc., Ontario,Canada.

The position measurement device 20 is described in greater detail in anumber of publications, including U.S. Pat. No. 5,564,437 and UnitedStates patent application publication No. 2005/0101966 A1 by S.Lavallee, both of which are incorporated by reference in their entirety.

The relative position of the patient's bones, such as the patient'sfemur 2 and the patient's pelvis 4, can be determined and tracked byattaching reference bodies 100, 110, which include respective markers102, 112. Reference bodies can be attached to bones or tools using pinsor screws (104, 114) or various quick release mechanisms. The referencebodies can also be shaped in the form numbers (e.g. “1”, “2”, “3” . . .) or alphabetical letters, such as “F” for Femur, “I” for Iliac, “P” forpointer, and so on, so as to avoid confusion as to which reference bodyshould be attached to which bone or tool.

The tracked objects and there relative positions can be displayed on ascreen that is connected to the computer system 30. In an embodiment,the display is a touch screen which can also be used for data entry.

The position measurement device 20 includes a number of different toolsthat are used at different locations and perform different functions asthe system 10 is operated to yield optimal osteoplasty data andinformation. These tools include the above described markers 50, whichact as landmark markers, as well as other tools, such as a milling orburring device 70 having at least three markers 72, which is an exampleof an object trackable by position measuring device 20. The system alsoincludes a pointer 120, with markers 122, which can be used to digitizepoints on the surfaces of the femur 2 and pelvis 4.

The burring device 70 also has a burr tip (drill tip) 74 having a knownspatial shape and relationship relative to markers 72. Positionmeasuring device 20 determines the position and orientation of markers72 in the three dimensional coordinate system 40. Based upon the knownspatial relationship between burr tip 74 and markers 72, the positionand shape of burr tip 74 is determined.

Computer 30 is preferably configured to allow at least one ofintra-operative 3-D shape model reconstruction and medical image data,like fluoroscopic images and ultrasound data, to be used in planning ofan area to be burred in a bone. The depth of the bone being removed bythe burring device 70 can be monitored and displayed by the computer 30.Thus, the burring depth can be compared to the previously plannedburring depth to allow the practitioner to minimize deviations betweenthe actual procedure and the preoperative plan. In one embodiment theburring device 70 is guided to allow the computer 30 to control theburring depth.

As previously mentioned, the plurality of reference bodies 100, 110, areused for determining the position and orientation of an individual'sbone in the three dimensional coordinate system 40. The reference bodies100, 110 are preferably rigid and include respective markers 102, 112,which are preferably configured to emit energy. Each reference body 100,110 preferably includes a respective attachment element, such as pins orscrews 104, 114, with which the reference bodies can be releasablyattached to a bone. For example, the reference body 100 is shown asbeing attached to femur 2. The position and orientation of femur 2 canbe determined based upon the position and orientation of markers 102attached thereto. Markers 102, 112 are sufficient to establish theposition and orientation of the rigid bodies 100, 110 within thecoordinate system 40.

Landmark Digitization

The system 20 also includes the pointer 120 and endoscope 130, whichcooperate to allow a practitioner to digitize landmarks of the femur 2and pelvis 4. Digitizing a landmark comprises determining the positionof the landmark in the three dimensional coordinate system, as discussedbelow. The pointer 120 includes the markers 122, which allow theposition and orientation of the pointer 120 to be determined in thethree dimensional coordinate system 40. The pointer 120 preferablyincludes a pointer tip 124 having a known spatial relationship to themarkers 122. Based upon the known spatial relationship, the position ofthe pointer tip 124 can be determined from the position and orientationof the markers 122.

In an embodiment of the invention, landmark points and or directions aredigitized with respect to the femur 2 and pelvis 4 with the pointer andare stored in the computer. Preferably, an anatomical coordinate systemfor the femur and the pelvis is defined based on at least a portion ofthe acquired data. The coordinate system could also be defined at leastpartially using kinematic methods, such as fitting a sphere to atrajectory (e.g. fitting a sphere to the trajectory of the femur as itis moved relative to the pelvis). From these trajectories points orplanes can be computed that correspond to the anatomy, such as thekinematic hip center. Possible landmark points for the femur include butare not limited to the femoral medial and lateral epicondyles, thecenter of the patella, a point acquired on the tibial crest with the legin flexion, the most lateral point of the greater trochanter, hipcenter, femoral head bone surface areas, etc. For the pelvis, pointssuch as the homo-lateral or contra-lateral anterior superior iliac spine5, anterior horn, points on the acetabulum lip, etc., could be used.

System User Interface

Now referring to FIGS. 2-4, the osteoplasty system 10 is preferably anintegrated system in which each of the tools is in communication with amaster controller, such as the computer 30, which serves to collect allof the data from the individual tools and then process the data tocalculate various measurements that are displayable to the physician.The osteoplasty system 10 accordingly includes a user interface 200(FIG. 1) that is supported by software in the computer 30. The userinterface 200 is configured to assist and walk the physician through theosteoplasty procedure to obtain optimal results and to assist thephysician in determining what the best course of action is in terms onproviding and optimizing the range of the motion of the hip.

A main user interface page or screen is preferably displayed on thedisplay 32 (FIG. 1) of the computer 30. The main user interface screencan include a section where the patient's name or other identifyinginformation can be entered using a keypad or the like. The main screencan also include a hip indicator box or region which the physician oroperator can highlight whether the procedure is being performed oneither the left hip or the right hip. Furthermore, the main screenincludes indicator boxes in which the physician or operator canhighlight whether the procedure is being performed on the femur,acetabulum, or both. The main screen includes a region where toolrelated information can be entered, as by highlighting a “sphericalburr” box, etc. The main screen also includes a section where the burrdiameter can by entered with keypad. The system can also allow the userto select a burr geometry and size from a database of burring toolsstored in the computer memory 30. The main screen further includes asection or region where different operations to be performed during theprocedure can be selected by the physician. For example, a region canlist a number of different options that can be performed to test thestate and kinematics of the hip, especially the range of motion thereof,to assist in diagnosing a hip condition, such as a zone in the hip inwhich there is a potential impingement of the femur with the acetabulum.

In addition, a screen can be provided to check the visibility of thereference bodies and in particular, one or more viewing areas can bedisplayed on the screen 32 (such as one area relating to depth of sightand another area relating to a radius of sight). If the object to whichthe reference body is attached is visible to the system, then an icon,such as a letter or number, will appear and be displayed and be visiblein one or more of the viewing areas. It will be appreciated that theobject should be visible in all viewing areas to ensure proper workingof the system.

Pointer & Burr Calibration

Before the pointer 120 is used to determine any position taken by itstip 124, the positioning measuring device 20 is calibrated. For example,the pointer 120 is calibrated by placing the pointer tip 124 in a conethat is integrated into reference body 110. The position of the conerelative to reference markers 122 is known. Therefore the position ofthe pointer tip 124 relative to markers 122 is now measured and storedin the computer 30. The pointer 120 fitted with markers 122 interactswith the detection system so as to precisely determine any positiontaken by its tip 124. Similarly, a spherical burr tip (drill tip) 74 ofthe burring device 70 can be calibrated by selecting precisely theexternal diameter of the burr tip 74 and then placing the burr tip inthe calibration cone. Alternatively, the axis and the tip of the burrcan be calculated using known methods. If the burr tip 74 has a knowncomplex shape which is not spherical at the bone contact, its exactshape is entered into the computer memory and many variations of thecalibration process can be developed to take the exact shape of the burrtip 74 into account accordingly.

Attachment of Rigid Bodies

To perform computer assisted osteoplasty surgery, especially femoralacetabular impingement, using the present system 10, the reference body100 is attached to the femur 2 at a selected point, as by screwing thereference body 100 into the bone with screw 104. The reference body 100includes the first set of at least 3 reflective markers 102 that arethus fixed to the femur 2 at the point. Similarly, the reference body110 is attached to the pelvis 4 at a selected point by screwing the body110 into the bone with one or several screws 114. The reference body 110includes the second set of at least 3 reflective markers 112 that arelikewise fixed to the pelvis 4, if the acetabulum option has been chosenat the beginning of the procedure. Since the reference bodies 100, 110are fixed to the bones 2, 4, the pointer tip 124 can be used to point atany given point on bone 2, 4, the position of which is precisely takenby its tip 124. Then, it is possible using a conventional dataprocessing system to determine the vector connecting the point where therespective bodies 100, 110 attach to the bones 2, 4 to the point of thetip 124, and therefore, locate any given point for any position of thefemur 2 or pelvis 4.

There is a plurality of methods in which an anatomical coordinate systemcan be setup by the surgeon. According to one exemplary method, a numberof different reference points are obtained. For example, the pointer tip124 is used to digitize the medial and lateral femoral epicondyles andthe most lateral point of the greater trochanter by placing the tip 124at these bone location which is determined and stored by positionmeasurement system 20. The midpoint between the medial and lateralfemoral epicondyles can be used for the knee center. Alternatively, apoint digitized on the center of the patella with the knee in flexioncan be used. The center of the hip can also be found by rotating thefemur with respect to the pelvis and measuring the relative motion ofthe bones as the surgeons manually manipulates the leg. The hip centerin the femoral reference frame can also be found without attaching areference body to the pelvis by known means. One such known method is tosearch for the point in the femoral reference frame that has a minimaldisplacement in the reference frame of the camera. A point on theanterior portion of the tibia (such as the tibial crest or ankle center)with the leg flexed can also be acquired in the formal reference frame,and combined with the knee and hip center, used to construct thesagittal plane of the femoral coordinate system. The mechanical axis isdefined as the axis joining the hip and knee centers. The frontal planecan be defined as a plane perpendicular to the sagittal plane andcoincident with the knee and hip centers. The transverse plane can bedefined as the plane perpendicular to the frontal and sagittal planesand coincident with the hip center. These points can be used easily tobuild an anatomical 3-D coordinate system attached to the femur.Similarly for the pelvis, one can use 2 points on the pelvis iliaccrests and 1 point taken on the symphysis to create a 3D anatomicalpelvis coordinate system. Many variations are possible.

Acquisition or data points are taken for the femur bone 2 (FIG. 2) andmore particularly, the pointer 120 is moved along the surface of thefemoral head of the femur bone 2 in order to measure the size of thefemoral head, which can be approximated as a sphere. A picture of asphere is preferably shown on the screen during the acquisition, withthe acquired points being super imposed on the sphere in real time asthey are acquired. The size of the sphere can also be changing in realtime according to points acquired by continuously fitting a sphere tothe acquired points. Thus, when the size of the sphere has grown stableand no longer changes, acquisition can stop automatically and theinterface can progress directly to the next screen. Next, additionalselected surfaces of the pelvis 4 and the femur 2 are investigated anddata points are taken and more particularly, the acetabular rim surfaceof the pelvis 4 is digitized using the pointer 120. FIG. 6 shows acollection and representation of a number of points that have beendigitized on the acetabulum lip.

In all of these steps, the digitization occurs by locating and logging aselected bone surface point in the three dimensional coordinate system40 and by using the data processing system. In other words, the pointer124 is moved along the bone surface and coordinate data are gathered foruse in generating a morphological 3D model of that bone portion.

As previously mentioned, the pointer 120 can be actuated to begin dataacquisition by simply activating an actuator, such as a foot pedal inwhich case it is merely depressed by the physician. The above describeddigitizing process can be thought of as a landmark acquisition process.Thus, according to an embodiment of the present invention, landmarkpoints and/or directions are digitized with respect to the femur 2 andthe pelvis 4 utilizing the pointer 120 and are stored in the computer30. There are a plurality of methods to set-up anatomical coordinatesystems on the bones. Preferably, an anatomical coordinate system forthe femur 2 is defined based on at least a portion of the acquired data.For the pelvis an anatomical coordinate system is defined by finding acorrespondence between the digitized landmark points and correspondingpoints on a deformable model of the pelvis, described in more detailbelow. The model of the pelvis includes the transformation between thedigitized landmark points and the anatomical coordinates system. Thus,the anatomical coordinates system of the pelvis 4 is known. Thecoordinate system can also be defined at least partially using kinematicmethods, such as fitting a sphere to a trajectory (e.g. fitting thesphere to the hip motion trajectory). As discussed in some detail above,possible landmark points include, but are not limited to, the femoralepicondyles, the most lateral point on the greater trochanter, the hipcenter, the homo-lateral or contra-lateral anterior superior iliacspine, anterior horn, points on the acetabulum lip, etc. The dataacquisition points can be used to determine other information, such as,locating a mechanical or neck axis of the femur 2. Additionally, whenmore accuracy is needed, the ultrasound probe 80 can be used to digitizeanatomical or landmark points, as described in more detail below.

Echomorphing

In some surgeries, such as arthroscopic surgeries, it is advantages toplace the portals in a particular position on the patient 1, relative tothe joint being operated on, to maximize visibility of the joint withthe endoscope and minimize trauma to patient. Prior to making theincision into the joint, ultrasound data can be acquired with ultrasoundprobe 80. The ultrasound probe 80 has a reference body 82 attached andcan be tracked in space by camera 49 and position measuring device 20.The position of the ultrasound image plane is precalibrated orcalibrated directly in the operating room and known with respect to thereference body 82. The relationship between ultrasound image pixel sizeand location, and real distance in mm is also precalibrated orcalibrated directly in the operating room. Thus, the position of theultrasound image plane, and any pixel the ultrasound image plane, isknown in reference coordinate system 40. Such uses of ultrasound orechographic probes are known and can be found in a number of documents,including U.S. Pat. No. 5,447,154, which is hereby incorporated byreference in its entirety. The ultrasound probe 80 is connected to thesystem computer 30 and thus the ultrasound images can be transferred andprocessed by the system computer 30 and displayed on the screen 32.Thus, ultrasound images can be recorded relative to bones 2 and 4, andstructures, such as, bone surfaces or soft tissues can be identified inthe images and thus their positions are known in reference coordinatesystems 40 and the bone anatomical coordinate systems. Theidentification of tissue structures such as bone surfaces can beperformed automatically by the navigation computer 30, using knownmethods as published in the literature, such as active contoursegmentation methods or ‘SNAKES’. Some examples of such previous methodsused to identify bone surfaces in ultrasound images can be found in thefollowing documents:

A Fully Automated Method for the Delineation of Osseous Interface inUltrasound Images” by Vincent Daanen, Jerome Tonetti, and JocelyneTroccaz, in Medical Image Computing and Computer-AssistedIntervention—MICCAI 2004: 7th International Conference Saint-Malo,France, Sep. 26-29, 2004 Proceedings, Part I, Editors: ChristianBarillot, David R. Haynor, Pierre Hellier, pages 549-557, LNCS, Springer2004;

Ultrasound Registration of the Bone Surface for Surgical Navigation” byD. Amin, T. Kanade, A. M. DiGioia III, and B. Jamaraz (Computer AidedSurgery Vol. 8, No. 1, January, 2003, pp. 1-16); and

Computer Understanding Bone Responses in B-Mode Ultrasound Images andAutomatic Bone Surface Extraction Using a Bayesian ProbabilisticFramework” by Ammet K. Jain and Russel H. Taylor, Medical Imaging 2004:Ultrasonic Imaging and Signal Processing published by Walker, WilliamF.; Emelianov, Stanislav Y., Proceedings of the SPIE, Volume 5373, pp.131-142 (2004). Each of the above cited references is herebyincorporated by reference in its entirety.

Doppler or color doppler analysis of the ultrasound images can also beused to identify the location of vessels. In addition, three dimensionalmodels of the bone surfaces can be reconstructed using the segmentedbone surface contours in the ultrasound images and deformablemorphing-based methods, as described in more detail below. It is alsopossible to check automatically that the segmented data on ultrasoundimages are consistent with an a priori approximate model of the bone andtherefore to reject false data accordingly and then to reiterate theprocess. It is also possible to ask the user to collect more images inanatomical areas where the model needs to be reconstructed in 3dimensions with some predefined degree of accuracy. Using these imagesand models, the surgeon can navigate the position of various tools suchas the pointer or a scalpel in order to place the portals or to make theincision precisely with respect to the joint or other identifiedanatomy, such as the femoral head/neck junction.

Bonemorphing

In an embodiment of the present invention, three dimensional geometricalsurface models of the bones are provided by image-free means.Preferably, these models are obtained by adjusting a deformable model ofthe bone to points acquired on the bone surface. Examples of some knownmethods of carrying out this task can be found in the followingreferences:

Building a complete surface model from sparse data using statisticalshape models: application to computer assisted knee surgery” by M.Fleute and S. Lavallee, published in Medical Image Computing AndComputer-Assisted Intervention—MICCAI'98, Spinger-Verlag LNCS Series,pages 880-887 October 1998;

Fleute M. Lavallee S, Julliard R. Incorporating a statistically basedshape model into a system for computer-assisted anterior cruciateligament surgery. Medical Image Analysis. 1999 September; 3(3):209-22;and

United States patent application No. publication 2005/0101966 A1 by S.Lavallee.

Each of the above listed references is hereby incorporated by referencein its entirety. Other known methods of obtaining geometrical bonemodels in surgery exist however (for example, matching medical imagedata such as CT, MRI, etc, to points acquired with a pointer or anultrasound probe). Another way is to morph a statistical model toinformation obtained by a few X-ray or fluoroscopic images, such as isdescribed in French Patent Publication No. FR1222636 entitled‘Reconstitution de surfaces en trois dimensions par utilisation demodeles statistiques’ by M. Fleute and S. Lavallee, which is herebyincorporated by reference.

In particular, the three dimensional shapes of the involved bones may beprovided with image free techniques, such as, using bone morphingsoftware which is capable of extrapolating very few range data to obtaina complete surface representation of an anatomical structure (e.g. abone). The specific details of bone morphing are set forth in the abovereferences but in general, a complete surface model is built from sparsedata using statistical shape models. In particular, the model is builtfrom a population of a number of specimen (points), such as femur orpelvis points, that are digitized and collected (See FIGS. 2 and 6).Models obtained from pathologic specimens may also be used. Data setsare registered together using an elastic registration method (e.g. theSzeliski and Lavallee method) based on octree-splines. Principalcomponent analysis (PCA) is performed on a field of surface deformationvectors. Fitting this statistical model to a few points is performed bynon-linear optimization. The different methods and algorithms can becombined and weighted respectively to create an optimal result in agiven situation. Similarly the parameters of the algorithms must betuned and adjusted to different situations. Results can thus bepresented for both simulated and real data. This method is very flexibleand can be applied to any structures for which the shape is stable.

In an embodiment of the invention, representations (bone morphs) of thefemur and pelvis bones, or a portion thereof, are displayed on thescreen 32. These models move on the screen 32 in real time based on themotion of tracked femur 2 and pelvis 4. In an embodiment of theinvention, the system guides the surgeon in manipulating the hip, byusing the bone representations as visual aids. Hip manipulations arepreferably preformed to identify joint motion ranges.

In other words, the present system preferably is configured to collectdata with the pointer 120 or ultrasound probe 80 in order to performbone morphing operations or procedures to thus obtain three-dimensionalgeometrical surface models of the bones. FIG. 2 shows representativebone morphing for the femur 4. Preferably, the representation of thebone on the display 32 of the computer 30 includes shading or some othertype of indicator that highlights the areas of the bone that haveincreased accuracy due to the physician having collected a number ofdata acquisition points in this region. The areas where more data pointswere collected are shaded differently or displayed in a different colorand the accuracy of the bone morphing (i.e., femoral bone morphing) onthe cartilaginous and bony surface can be checked (as by highlightingicons that are displayed). Areas of the bone that have not been scannedusing the pointer 120 are simulated using conventional extrapolationtechniques as commonly found in bone morphing techniques and systems.

Accordingly, target locations of the femur 2 are digitized and asillustrated in FIG. 2, the neck and head of the femur 2 are preferablydigitized as shown in FIG. 2. On the display screen, the systempreferably indicates where the pointer 120 has traveled and collecteddata acquisition points as by providing a visual colored pointrepresentation on the screen. Bone morphing for the femur 2 isperformed. Shading or some other type of indicator can be used tohighlight the areas of the bone that have increased accuracy due to thephysician having collected a number of data acquisition points in thisregion.

Target locations of the pelvis are digitized. On the display screen, thesystem preferably indicates where the pointer 120 has travelled andcollected data acquisition points as by providing a visual colored pointrepresentation on the screen. In particular, zones which are suspectedto be potential areas of impingement on the acetabular rim are digitizedand morphed. Collecting a large amount of points in these areas willassure a high accuracy. Bone morphing for the pelvis is performed.Shading or some other type of indicator can be used to highlight theareas of the bone that have increased accuracy due to the physicianhaving collected a number of data acquisition points in this region.

The collection of bone surface points can be also performed using theburring device 70 directly without using a specific extra pointer.

The accuracy of each of the bone morphing of each of the femur and theacetabulum can be checked using the pointer 120.

Fluoroscopy

Although the system does not strictly require the use of an ionizingimaging means, a tracked C-arm (not shown) or other intra-operativeimaging modality could be also connected to the system and fluoroscopeimages can be acquired and displayed on the navigation screen. Inaddition, 3D C-arms such as the ISO-C 3D system marketed by Siemenscould also be used to display a reconstructed image volume. Theconnection and use of such imaging devices to CADS systems is known(see, for example, U.S. Pat. No. 6,697,664 by Kienzle et al., which ishereby incorporated be reference in its entirety). Images of the patientanatomy can be acquired and displayed on the navigation screen 32, andthe position of calibrated and navigated tools such as the pointer 120with tip 124 can be displayed and projected overtop of the acquiredimages. Anterior-Posterior and lateral views can be taken and used toaid the surgeon in selecting points on the bone or soft tissue anatomy,such as the most lateral point on the greater trochanter of the femur,or the entry point of an arthroscopic portal. Anatomical points can alsobe entered into the system by selecting them directly on the imagesusing the touch-screen 32.

The computer system can also contain an algorithm that uses the imagesto assist morphing of the bone model. For example, any algorithm can beemployed to extract the bone contours on the images, such as thealgorithm commonly known as the ‘Canny’ edge detector, or any activeSNAKE algorithm. The segmented contours can be used as inputs into theBoneMorphing algorithms, to assist the surgeon in acquiring sufficientsurface data for Morphing. These contours could supplement or evenreplace the surface digitization steps with the digitizing probe 120.The Morphing model can be used to aid the segmentation algorithm in aniterative process. For example the morphing model could be used toguaranty the robustness of the segmentation by helping to assess whetheror not a segmented image contour really lies on the surface of the boneand is not a false positive. Shape similarity measures or deformationenergy measures or intensity gradient measures could be used alone or incombination to relate the process. Alternatively, landmarks such as thefemoral head center and surface and most lateral point on the greatertrochanter can be identified on the images to determine the initialattitude of the bone model, and then an automatic morphing process canbe launched which deforms the model until it matched the silhouette ofthe bone on the images. As mentioned above intensity gradientinformation or edge detector techniques can be used to match the modelto the images. See French Patent Publication No. FR1222636 and U.S.patent application Ser. No. 10/088,772, which are hereby incorporated byreference in its entirety. The process can also be semi-automatic thesurgeon can input information on the touch-screen. For example, he canidentify boundary areas where the bone lies, or points on the bonecontours, or regions where the morphing is inaccurate and does not matchthe images (for example, if the algorithm has fallen in a local minimadue to a falsely detected contour), in which case the morphing can berefined in those areas. Bone pins 104 or reference markers 102 in theimages can also be identified and excluded from the morphing algorithm.

At any time the morphing model can be superimposed over the bone images,allowing the surgeon to gauge the accuracy of the morphing. Multipleviews can be shown. As will be described in more detail later on, whenthe bone surface is altered with a burr, the surgeon can directlyvisualise how the bone has been altered and much bone has been removedby comparing the remorphed surface to the underlying image of theoriginal unaltered bone. Calibrated probes 120, burrs 70, arthroscopes130, scalpels and instruments can all be visualised on the screenovertop of the images and relative to the bone model.

Clock Display

Referring now at FIG. 9, two views of the femoral model are displayed,with a clock reference 400 including contours projected on the 3D bonesurface corresponding to 12′ (412), 11′, 10′, 9′ (409) . . . 6′ (406), .. . 1′ and so on. Contour lines are divided radially from the clockcenter with equal spacing, as in a real clock. The clock contours aresuperimposed onto the femoral and acetabular bone surfaces to help thesurgeon know where he is on the femoral surface and to know where theposition of his tools are relative to the anatomy. The center of theclock reference can be defined on the femur and on the acetabulum byusing the center of the hip, as described above. The orientation of theclock reference can be defined on the femoral head and neck by selectionof an anatomical point with pointer tip 124 that corresponds to aparticular hour on the clock, such as 12 or 9 or 6 o'clock. Theanatomical coordinate planes can also be used in combination with apoint (for example the frontal plane in combination with the mostproximal point in an area of the femoral neck, head, head-neck junction,or greater trochanter). Such points can be selected with the aid of thefluoroscopic or echographic images displayed on the screen.

For example, using the anterior-posterior fluoroscopic view, the surgeoncan place his probe on a point on the femoral neck or head correspondingto the 12 o'clock position, and can verify on the images that this pointcorresponds to the most proximal point in the anterior-posterior X-rayview, or the midpoint of the neck in the lateral view. Similarly, areference point can also be chosen on the acetabulum, as the mostproximal summit point on the acetabular rim.

The system preferably suggests a clock orientation intelligently andautomatically, and allows the surgeon to check and validate or adjustthis suggest orientation as he deems necessary. One method of definingthe clock reference automatically for the femur is to use the anatomicalcoordinate system defined by the mechanical axis of the femur (ie thevector joining the center of the femoral head and the center of the kneeand determined previously). In particular, the most proximal areas ofthe femur can be determined by searching for particular points in themodel that have the highest coordinate the proximal direction. Themechanical axis can be projected through the femoral head surface todetermine one point on the 12′ contour. Other information can be used todetermine the direction of the contours, such as the most lateral pointselected on the femoral trochanter, or the frontal plane direction.Similarly, maximum dimensions in particular directions can be used (suchas in the anterior and posterior directions in the area of the neck) andaverages of all points can be taken to construct the clock hourreferences.

Alternate representations of the clock can be envisioned. For example,in a lateral view of the femur or in a view perpendicular to the rimplane of the acetabulum, a ring can be displayed around the bone that isdivided in to radial sections and labelled with the clock hours. Thehour corresponding to a particular tracked tool position such as theprobe 124 or burr tip 74 can be highlighted when it is visible. Thus asthe surgeon is burring from one hour to another the highlighted hour onthe clock ring changes and he can easily detect when he has changedlocations on the bone surface.

Calculating Impingement

The femur 2 can be manipulated with respect to the acetabulum, and themodels on the screen 32 move in real time as the surgeon manipulates thejoint (See. FIG. 8). In particular, the surgeon can move the joint untilhe sees the bones impinging upon themselves on the models. Thisimpingement corresponds to an actual and real impingement of motion asfelt by the surgeon when he manipulates the leg. In addition, thecomputer can detect the instant when the bone morphing models come intocontact with each other, and this can be alerted to the surgeon by achange in color on the display 32, for example. Thus the surgeon canverify that impingement felt is a real bone impingement and not one dueto other causes, such as, for example, due to a soft tissue contractureon the other side. The surgeon can also verify the exact area ofimpingement on the bones, and this can be automatically stored in thecomputer. Collision detection algorithms for geometric models are wellknown, and any such algorithm could be used. The following article givesan example of a fast algorithm for calculating range of motion andimpingement in the hip joint, and is hereby incorporated by reference:Real-Time Estimation of Hip Range of Motion for Total Hip ReplacementSurgery by Kawasaki et al, Published in Medical Image Computing andComputer-Assisted Intervention—MICCAI 2004: 7th International ConferenceSaint-Malo, France, Sep. 26-29, 2004 Proceedings, Part II, Editors:Christian Barillot, David R. Haynor, Pierre Hellier, pp. 629-636, LNCS,Springer 2004. United States Patent Application publication No.2003/0176783 A1, for example, discloses a fast algorithm for detectingimpingement between implants and/or bones using implicit object modelsfrom reconstruction of anatomical CT data, and is hereby incorporated byreference in its entirety. Such an algorithm could be used withintra-operatively obtained models, such as, the Bone Morphing modelsdescribed above. One or several contact positions can be stored in thecomputer, but for clarity we will consider only one contact position andthan repeat this process for any positions needed. The contact positioncan be related to a reference rotation, such as 0 degrees, for example.

Relationship Between Joint Rotation and Milling Depth

There are a plurality of methods that the system can employ to aid thesurgeon in removing the correct amount of bone in order to restoreproper joint function and range of motion. In an embodiment, thecomputer aided osteoplasty system establishes a relationship between theincrease in joint motion and the milling depth with respect to the bonesurface. In particular, the computer can simulate a rotation of thefemur 2 into the pelvis 4 (or vice versa), for a predetermined amount orrotation, in any particular direction. Such a rotation can be about thehip center, for example, which has already been determined as describedabove. The direction of the rotation can be defined by a plurality ofmethods, such as, for example, by the surgeon, using the anatomiccoordinate system. For example, he could enter 10.degree. of flexion or10.degree. abduction on the tactile screen. Alternatively, he couldphysically move the femur 2 in a particular direction, and the systemcan record a trajectory of motion and calculate the direction tosimulate the rotation in. The system can also calculate the direction asa function of the impingement area, such as a normal vector the surface.The system can simulate motion on one bone respect to another bone usingeither methods and display this motion on the screen for the surgeon toevaluate. By simulating a rotation of the femur 2 into the acetabulum,the intersection of the two surfaces can be calculated for a discretenumber of rotations, for example, for every one degree from zero to tendegrees. Alternatively, the surgeon can enter into the computer 30 atarget amount of motion that he would like give to the patient, and thesystem will automatically calculate the intersection of these surfacesfor the given target rotation.

Referring now to FIGS. 3A-D, FIG. 3A schematically depicts a femur 301and acetabulum 302 with hip center 300. The hip center location is knownin both the femoral and acetabular coordinate systems. For joints whichare particularly lax, or for example, when the hip is distracted using atraction table, the hip center 300 is no longer coincident for the femur301 and acetabulum 302. The center 300 could be transferred from onebone to another one for a stable position of the joint, for example,when it is compressed, by taking a measurement with the camera 49 ofposition measuring device 20. FIG. 3B schematically depicts the femurrotated relative to the acetabulum by a target number of degrees‘.alpha.’. The corresponding intersection of the two surfaces 305 isidentified using their locations. This defines a relationship betweenthe target improvement in range of motion and the volumetric amount andlocation of bone volume to be removed in order to achieve that targetgoal for either the femur 301 or the acetabulum 302. This intersectingsurface or volume 305 can be displayed on the screen 32 as a differentcolor or different intensity of color on the bone, in order to indicatewhich areas of bone the surgeon must remove in order to achieve thetarget increase in range of motion.

FIG. 3C schematically shows how a series of surfaces can be determinedthat correspond to a range of increased rotation values. Each surface inthe series of surfaces 310 can correspond to an additional one or twodegrees of hip motion, for example. These surfaces or impingement mapscan be displayed as a color map on the display screen 32, with differentcolors representing different degrees of rotation. A color key can alsoindicate which colors correspond to which degrees of rotation or millingdepths. In cases where the surgeon would only like to perform theosteoplasty on one side of the joint, for example, on the femur 301, thecolor map can be displayed only on the bone to which the osteoplastywill be preformed.

A color map can also be calculated and displayed for one bone withoutinformation on the shape of the second bone. This can be very useful,for example, when the osteoplasty is to be performed only on one side ofthe joint, and the practitioner chooses to only attach a reference frameto the bone in which the osteoplasty is to be performed. Thepractitioner could use the probe 120 or calibrated burring device 70 todefine a potential impingement zone, such as a center point of theimpingement area or a border around the impingement area, as shown inFIG. 4. Each point, triangle, or pixel in the bone model could berepresented with a color that is dependent on the depth or distance tothe surface of the bone. Again, the color spectrum could be a discreteone or a continuous one. Transparency could be used for the bone area orthe color map area for clarification. The relationship between any pointin the bone and the increased range of hip motion can also be calculatedas a function of the distance from the point to the hip center 300, forexample, and from the point to the bone surface, as illustrated FIG. 3D.A trigonometric function such as tan(beta)=d/(mag/PT−HC)), where PT isthe point or pixel to be colored, HC is the hip center point 300,mag(PT−HC) is the distance between these two points, and d is thedistance between PT and the bone surface. The distanced can be theclosest distance to the bone surface, or the distance calculated inspecific direction. The current resection depth or potential increase injoint Range of Motion (ROM) can be displayed on the screen in real time.

By tracking the burring device tip 74 (which has a known surfacegeometry and size and position with respect to its reference marker 72)with respect the femoral bone surface during the milling action, thesystem can register and track the volume of bone that has been removed.The system can thus display the tool and show the bone as it is beingremoved in real time, to reveal the color or impingement map informationdisplayed on the newly burred bone surface (FIG. 5). The surgeon canthan know to stop milling once he or she reaches a particular color ofbone in the color map. This target color could be the same color as thewhole bone itself, or any other color. This simulation of bone removalcan carried out using voxel based methods as known in the literature,for example, or any other rendering algorithm available.

The bone surface can be recomputed or remorphed at any phase during thesurgical procedure using the pointer 120 or the burring device 70. Thesurface and geometry of the milling device 70 is known, and can be usedas a pointer to remorph the surface. An example technique for carryingout this using a spherical shape is described in United States patentapplication publication No. 2005/0101966 A1, which has already beenreferenced. This can be easily extended to other shapes, such ascylinders and elliptical shaped tools, simply by modifying thegeometrical or parametric model accordingly (or example, by making thetool radius a function of the axial position). This surface or thesuperposition of all the recorded tool positions measured and storedduring the milling phase can thus be used to remorph the bone surfaceshape, at any time during the procedure.

Note if the impingement map is dependent on the shape of the bonesurface, the impingement map can therefore also be updated at any timeduring the procedure. For example, as described above and illustrated inFIG. 3C, the impingement or color map can be recomputed for the femur301, as the acetabular surface 302 is being shaved and remorphed. Thiscan be happening in real time in the background so as to be transparentto the surgeon. The impingement or color map can also be recomputed inreal time or at any particular time during the procedure, for the casein which the color map is calculated as a function of the bone beingburred, as described FIG. 3D or and/as above. Thus the impingement orcolor map can be a dynamic one.

In one preferred embodiment of the invention, the following process canalso be applied. Once both potential infringing shapes on femur andpelvis have been acquired, the surgeon can select the amount of extrarange of motion he would like to gain. The computer can then extrapolatethe femur motion up to that desired position and computes theintersection on both surfaces. The intersection of surfaces on the femurcreates a patch surface that is connected to the rest of the surface ofthe femur. This new surface results in an objective to reach for thesurgeon. A color map indicates the distance between the current surfaceand the objective surface on each point of the surface. For instance,red indicates more than 5 mm, dark orange between 4 and 5 mm, lightorange between 3 and 4 mm, yellow between 2 and 3 mm, green between 1and 2 mm, grey between 0 and 1 mm and blue for negative values. When thesurgeon uses the burring device 70, the color map changes in real timeand colors progressively vanish until the surgeon reaches a grey or bluecolor map everywhere, which indicates that the objective surface hasbeen reached. Such a representation could be used in cases where thecolor map of the femur is dependent only on the shape of the femur (i.e.color at any point is proportional to the distance to the originalfemoral neck surface).

The range of motion can also be a number in degrees of flexion that iscomputed, displayed and updated on the screen for the surgeon at anytime and in real time. Indeed, for a given shape of the bone, it ispossible to extrapolate the degree of motion of the femur and calculateat what angle the impingement is reached. This new value represents thedegree of motion that the surgeon wins each time the surgeon is millingthe bone. Any method can be used to calculate the range of motion, suchas those described and referenced above.

All previous modes can be combined together.

The present system easily permits the surgeon to use the pointer 120 anddelineate the femur surface (femur bump) that is to be shaved in orderto achieve the desired fit between the femur and acetabulum surface.Then as shown in FIG. 4 in the case of the femur bone and in FIG. 7 inthe case of the pelvis bone, the shaving tool (e.g. burring device) canUm navigated along the respective surfaces and the movements of theshaver are reflected in real time on the computer screen 32.

The navigation system 10 is an integrated system and can have theburring device 70 connected to it. Therefore, the system can control thefunction of the milling tool as a function the position of the tool withrespect to the bone as measured by the camera 49. For example, the burrmay be ‘pulsed’ as the surgeon approaches the boundary of theimpingement map, according to the target increase in range of jointmotion or milling depth. This can be accomplished by altering theelectrical supply signal to the burring device 70. Alternatively, theburr can be automatically stopped when it passes the boundary. Inparticular, the burr can be stopped or pulsed when its surface reachesthe target impingement surface as identified by the system. The actualburring can also be guided by a robot to obtain a precisely cut surfacewith respect to the impingement color map.

Flexible Protocol

One feature of the invention is that it is a flexible system and allowsthe surgeon to select the protocol that he wishes at the beginning ofthe procedure. Several options are possible, including the order of theacquisitions and steps, the instruments and optional imaging devices tobe used, and the bones to be operated on. The following is one exampleof a protocol that the surgeon can define:

Pins and Rigid Bodies installed after draping

Pointer and Milling tool calibration steps completed first (can beperformed by OR technicians)

Tracked C-arm images can be used to aid point acquisitions, morphing andshaving at any step

Symphisis÷Homolateral spine acquisitions

Tibial Crest+Patella acquisitions

Dynamic Hip center and Pre-Op Range of Motion (ROM) measurement

apply traction, make portals, clean bone surfaces (portal incisions canbe assisted using navigation C-arm images, using probe to mark desiredlocation on skin)

most lateral point on greater trochanter (optionally under navigatedfluoroscopy)

Femoral head surface acquisitions

Points on the acetabular rim

Acetabular surface digitization: BoneMorphing (BM) Acetabulum+ModelCheck

Adjust Acetabular clock position with one point if necessary

Acetabular Navigated Shaving

release traction

Femoral neck surface digit: BM Femur+Model Check

Adjust Femur clock position with one point

Femur Navigated Shaving

Post-Op ROM measurements

Visualization of any impingement zones-I-Display of increased ROM

Repeat any steps if necessary

Save report on CD-ROM, USB key or Network

The flexible protocol allows the surgeon to select the order and actionsand tools of his preference, and many variations of the above protocolcan be chosen. He can also save and store his preferred protocol under auser name of his choice. Modification of the options can also be carriedout during the procedure, in case for any unforeseen reason the surgeonwould like to change the order or jump to a different step. This isaccomplished touching a button on the touchscreen which brings up a listof options and workflow order, which can be modified using theinterface, allowing the surgeon to return to the new modified protocol.

The invention should not be limited to the description given above. Forexample, this system could be applied to the knee elbow or shoulder,joint or any joint for that matter. In addition, the invention could becombined with other such known features of computer assisted surgicalsystems, such as the calculation of joint laxity parameters, eitherlinear or rotational, in various directions such as those described inU.S. patent application Ser. No. 11/299,287, entitled “Computer AssistedOrthopaedic Surgery System for Ligament Graft Reconstruction” by P.Colombet et al., which is hereby incorporated by reference in itsentirety.

While the invention has been described in connection with certainembodiments thereof, the invention is capable of being practiced inother forms and using other materials and structures. Accordingly, theinvention is defined by the recitations in the claims appended heretoand equivalents thereof.

All above cited references are hereby incorporated by reference.

1. A method for performing computer-assisted orthopaedic surgecomprising the steps of: using a device to acquire first coordinatepoints on a surface of a first bone; using the device to acquire secondcoordinate points on a surface of a second bone that forms a joint withthe first bone; producing and displaying a three-dimensional geometricalsurface model of the first bone based at least initially on the acquiredfirst coordinate points; producing and displaying a three-dimensionalgeometrical surface model of the second bone based at least initially onthe acquired second coordinate points; moving the first and second bonesrelative to one another and detecting maximum amplitudes of rotationbetween the first and second bones; identifying a zone of impingementbetween the first and second bones on at least one of the bones;generating and displaying as a color map at least one surface of atleast one bone model, the at least one surface being within the zone ofimpingement, the color map including different colors representingdifferent depths of bone to be removed in order to achieve an increasedrange of motion between the two bones, the colors of the color mapindicating a distance between a current bone surface and an objectivebone surface for each point of the least one surface, the objective bonesurface being the bone surface that permits a target increased range ofmotion between the two bones to be achieved; using, a tracked tool toremove bone in the zone of impingement based on real time informationprovided on the color map; and moving again the first and second bonesrelative to one another and detecting an increased amount of rotationbetween the first and second bones.
 2. The method of claim 1, furtherincluding the step of: representing the different colors by a continuousor discrete spectrum of varying brightness of a single color.
 3. Themethod of claim 1, wherein the step of acquiring points on each of thesurfaces of the first and second bones comprises the step of: projectinga current pointer position overtop of registered fluoroscopic images ofthe bones.
 4. The method of claim 3, further including the step of:extracting bone contours from the registered fluoroscopic images toassist in generating the three-dimensional geometrical surface models.5. The method of claim 1, further including the step of: superimposingthe generated three dimensional geometrical surface models overregistered fluoroscopic images of the bones to allow a surgeon to verifymodel accuracy.
 6. The method of claim 1, further including the stepsof: continuously, in real time, updating the three-dimensionalgeometrical surface models of the bones as bone is removed; andsuperimposing the updated three dimensional geometrical surface modelsover registered fluoroscopic images of the bones to allow a surgeon tocompare the updated surface of the three dimensional geometrical modelswith the original surface models in fluoroscopic images.
 7. A method fordisplaying bone-related information to guide a surgeon in removal ofbone as part of a system used in computer-assisted orthopaedic surgerycomprising the steps of: producing and displaying three-dimensionalgeometrical models of first and second bones, the first and second bonesforming a joint; identifying a zone of impingement between the firstbone and the second bone on at least one of the bones; establishing arelationship between an increase in motion of the joint and a resectiondepth with respect to a surface of at least one of the first and secondbones that is to be removed within the zone of impingement; andgenerating and displaying a color map of at least one surface of atleast one bone, the at least one surface being within the zone ofimpingement, the color snap including different colors representingdifferent depths of bone to be removed in order to achieve an increasedrange of motion between the first and second bones.
 8. The method ofclaim 7, further including the steps of: continuously, in real time,updating the three-dimensional geometrical surface models of the bonesas bone is removed; and continuously, in real time, updating the colormap to show updated depths of at least one of the first and second bonesas bone is removed.
 9. The method of claim 7, further including thesteps of: moving the first and second bones relative to one another anddetecting maximum amplitudes of rotation between the first and secondbones prior to and to permit identification of the zone of impingement;and moving the first and second bones subsequent to one another afterremoval of bone in the zone of impingement and detecting an increasedamount of rotation between the first and second bones.
 10. The method ofclaim 7, wherein the first bone is a femur bone and the second bone is apelvis bone and the joint is defined by the femur and the acetabulumsurface of the pelvis bone, the range of motion being a rotation of thefemur into the acetabulum, with the zone of impingement being defined bythe femur and the acetabulum, and further including the step ofindicating on the color map different increased degrees of rotation withdifferent colors that represent different depths between a current bonesurface and an objective bone surface which results in a desiredincreased degree of rotation between the first and second bones beingrealized.
 11. A method for performing computer-assisted orthopaedicsurgery comprising the steps of: using a device to acquire firstcoordinate points on a surface of a first bone; using the device toacquire second coordinate points on a surface of a second bone forms ajoint with the first bone; producing and displaying a three-dimensionalgeometrical surface model of the first bone based on the acquired firstcoordinate points; producing and displaying a three-dimensionalgeometrical surface model of the second bone based on the acquiredsecond coordinate points; moving the first and second bones relative toone another and detecting when the geometrical models of the first andsecond bones come into contact with one another in a defined zone ofimpingement; generating and displaying as a color map at least onesurface of at least one bone model, the at least one surface beingwithin the zone of impingement, the color map including different colorsrepresenting different degrees of increased range of motion between thetwo bones, as well as different depths of bone to be removed in order toachieve the different increased range of motions between the two bones;selecting one increased range of motion that is desired between the twobones; and using a tracked tool to remove bone in the zone ofimpingement based on real time information provided on the color mapuntil the selected increase in range of motion is achieved as indicatedwhen the color map reveals a target color for at least the zone ofimpingement.
 12. The method of claim 11, further including the step of:representing different colors represent different degrees of rotationbetween the first and second bones with different colors.
 13. The methodof claim 11, further including the step of: representing each point ofeach model in a particular zone with a color that is dependent on adepth or distance between a current surface of the bone and an objectivesurface of the bone that results in the selected increase in range ofmotion.
 14. The method of claim 13, further including the step of: usingas part of the color map colors that are part of a discrete colorspectrum.
 15. The method of claim 13, further including the step of:using as part of the color that map colors are part of a continuouscolor spectrum.
 16. The method of claim 11, further including the stepof: representing greater depths of bone that need to be removed in orderto achieve the selected increased range of motion with darker colors ofthe color map.
 17. The method of claim 11, further including the stepof: indicating in the color map different increased degrees of rotationwith different colors that represent different depths between thecurrent bone surface on which the acquired points were taken and theobjective bone surface which would result in the selected increaseddegree of rotation between the two bones being realized, wherein therange of motion is a rotation of femur into an acetabulum, with the zoneof impingement being defined by the femure and the acetabulum.
 18. Themethod of claim 18, further including the step of: changing the colorsof color map in real time as a device is used to remove the bone suchthat the colors progressively vanish as more bone is removed until theentire color map in the zone of impingement has a first target colorwhich indicates that the objective bone surface has been reached. 19.The method of claim 11, further including the step of: using a pluralityof colors in the zone of impingement to represent different depths ofbone that are needed to be removed in order to achieve the selectedadditional range of motion.
 20. The method of claim 11, furtherincluding the step of: creating and displaying the color map for thefirst bone without information on a shape of the second bone.
 21. Themethod of claim 11, further including the step of: creating anddisplaying in real time, an updated color map showing a current depth ofbone as bone is removed in order to achieve the different increasedrange of motions between the two bones and to guide an operator of thetracked tool as to how much additional bone is to be removed, whereinthe color map is a dynamic color map.
 22. The method of claim 11,further including the step of: displaying at least the first geometricalbone model with a clock reference superimposed thereon and includingtwelve latitudinal lines projected on the three dimensional bone model,the latitudinal lines being divided radially from a center point withequal spacing to represent twelve hours of a clock, each latitudinalline being uniquely identified to assist a surgeon in identifying andconveying a particular position on the bone surface.
 23. The method ofclaim 23, further including the step of: highlighting a closestlatitudinal line relative to the tracked tool that is positioned on thebone surface to assist the surgeon in determining when the surgeon haschanged locations on the bone surface.